The religious wars in Europe during the early 16th Century to mid-17th Century between Catholics and Protestants. They resulted in the death of about one-third of the population of Germany and major loss of life in many other European countries.
The Nazi Holocaust which was a genocidal program directed primarily against Jews but with Roma, homosexuals, mentally challenged, and other victimized groups as well.
Frequent clashes, largely in the Middle East:
In the form of hate-motivated mass murder between followers of the Sunni and Shi'ite branches of Islam or
Between Muslims and followers of other religions in predominately Muslim countries -- often victimizing followers of other religions, primarily Christianity.
Quotations showing how people have sometimes treated others who hold different religious beliefs:
Anon: "We have lobbed verses of Scripture, like hand grenades, into the camps of others, convinced that we, alone, have truth."
Bill Clinton; during his 1997 Presidential Inaugural Address: "The divide of race has been America's constant
curse. Each new wave of immigrants gives new targets to old prejudices. Prejudice and
contempt, cloaked in the pretense of religious or political conviction, are no different.
They have nearly destroyed us in the past. They plague us still. They fuel the fanaticism
of terror. They torment the lives of millions in fractured nations around the world. These
obsessions cripple both those who are hated and, of course, those who hate, robbing both
of what they might become."
Blaise Pascal: "Men
never do evil so completely and cheerfully as when they do it from religious conviction."
From a Boston Globe editorial: "A religion does not create
murderers. The twisted human psyche does that when it forgets that the
bedrock of all faith is love." 1
Alan M. Dershowitz, an American lawyer, jurist, political commentator, and author of The Trials of Zion: "All religions and cultures suffer from sources that preach hate against the ‘other.’ Throughout history some have, tragically, practiced what their sources preached, while some have sought to dismiss or even counteract the hateful words of their sources."
About religious diversity:
The religious diversity in the world is immense.
According to David Barrett et al, editors of the "World
Christian Encyclopedia: A comparative survey of churches and religions - AD 30 to 2200," there are 19 major world religions which are subdivided into a
total of 270 large religious groups, and many tens of thousands of faith groups (denominations, traditions, etc.). 34,000 separate Christian faith groups have been identified worldwide. 2
In terms of quality of life, the most important of all religious topics may well be how people, religions, cultures, and countries handle religious diversity.
We have one world, with multiple religions. Each religion has multiple divisions that are commonly called denominations, traditions, etc. Each division is composed of many congregations. Each congregation has many members.
We don't necessarily all get along well together. Even within a single religion, many times people will disagree about religious beliefs and practices. Sometimes the way we handle diversity leads to rejection and conflicts. It may even escalate to mass murders, war, and genocide.